This invention relates to compressor pistons in general, and specifically to a method for coating a piston with a wear protection layer.
Air conditioning system compressors have, for some time, been made from aluminum alloy, as have the cylinder blocks within which the pistons reciprocate. The pistons are subject to significant side load, and it has been conventional practice to apply a wear protective coating, typically a mixture of a resin binder such as polyamideimide, a dry lubricant (such as PTFE) if desired, a suitable solvent and a neutral filler to round out the mixture to the piston. One conventional coating application method is a simple spray application, followed by a heat curing step and a final machining of the cured layer to bring the piston to size. Spray coating inevitably coats parts of the piston that don""t need it, barring an additional, labor intensive masking step. Also, in order to achieve a sufficiently thick coating layer in one spray coat, a thick coating must be applied, which is subject to sagging and running.
A well known alternative to spray coating the entire piston is to either screen print or roller coat only a defined area of the piston. In screen printing, a piston is rolled beneath and against a flat screen that has been provided with a xe2x80x9crolled outxe2x80x9d pattern corresponding to the coating pattern that is desired on the cylindrical piston. A supply of coating material on the top of the screen is forced by a squeegee through the screen and onto the surface of the piston, in a single 360 degree (at most) turn and in a single layer. In roller coating, coating material from an open pan is transferred by a metal roller to a hard rubber roller and printed from the hard rubber roller onto the piston workpiece, also in a single workpiece rotation and single layer. The wet coated pistons are transferred to an oven for later heat curing, which removes the solvents and leaves the solid, hardened coating behind. In most cases, the pistons must be ground to final size after curing, since the coating itself cannot be applied with sufficient precision in terms of its thickness. Therefore, extra coating thickness has to be applied in the first place. Examples may be seen in U.S. Pat. Nos. 5,941,160 and 6,283,012, each of which discloses the same basic roller coating apparatus, although the latter claims that the final grinding step is not needed. Shortcomings in these known techniques result both from the apparatus and methods themselves, as well as the coating materials.
The main drawback of known roller coating methods and apparatuses is the fact that, in order to achieve a sufficiently thick coat in a single layer that does not run or drip on the workpiece surface, the coating must be relatively thick and viscous. For example, the viscosity of the coating material in U.S. Pat. No. 6,283,012 is said to be between 5000 and 150,000 centipoise. Such a thick, one layer coating, regardless of viscosity, is still liable to running or dripping, necessitating that the piston continually be rotated to prevent differential gravitational effects on the thick, poorly adhered layer. The thick layer is also prone to bubbling in the curing oven, as the solvent is flashed out. Grit or sand blasting of the surface has generally been necessary as a pre treatment to the piston surface to get the coating layer to stick to the surface, also. Futhermore, there is a limit on how thick such a single layer coating can practically be applied, regardless of its high viscosity. Experience has shown that such a single layer can really only be reliably applied to a thickness of about 15 microns, despite claims to the contrary, and it is best if the coating layer, post grinding, is at least 35 microns thick. Even if such single layer coating methods work as well as possible, there is an inevitable area of overlap at the beginning and end of the layer that is thicker than desired, and even more subject to sagging. Another practical shortcoming is that the open pan of coating material used in conventional roller coating is continually both losing organic solvent to the surrounding air, and forming a skin on the top, which must be continually removed, and which causes variations in viscosity of the raw material.
As noted above, known coating materials and mixtures themselves have shortcomings. For example, the main coating constituent disclosed in one of the aforementioned patents is PTFE, which has a very high ratio relative to the polyamide resin binder, ranging from 0.8 to 3. Both patents stress the claimed importance of high levels of PTFE. However, PTFE is relatively soft with a low mechanical strength, and has a significantly higher coefficient of thermal expansion, as compared to the aluminum surface to which it must bind. A weak coating layer that is differentially thermally stressed at the part-layer interface is clearly not optimal or ideal. Furthermore, while PTFE may present some advantages in terms of dry lubricity when a compressor starts up, once in is running with a layer of refrigerant and entrained oil located in the piston-cylinder surface interface, high levels of PTFE can in fact jeopardize the ability of the oil to wet the surface.
The subject invention provides a method and apparatus for practicing the method that coats a piston or other cylindrical workpiece not in a single, thick layer, but which rolls on multiple, very thin layers in rapid succession, creating a final, thicker layer that adheres without surface pre treatment, and which is solid, even and strong.
Instead of applying a very viscous coating material directly out of an open pan, a less viscous coating material is maintained at the proper viscosity and homogeneity within a closed tank, preventing solvent loss and skinning. Coating material is pumped from the tank through a line and distributed by a nozzle to an engraved metal roller that distributes a controlled amount of coating material onto a larger diameter elastomer coating roller, in a pattern and to a depth determined by the etched pattern on the metal distribution roller. Beneath the coating roller, a heated manifold continually heats the coating roller to a temperature well below the coating curing temperature, but sufficiently high to flash off much of the solvent. Concurrently, a piston workpiece that has been pre heated (but not grit blasted or otherwise pre treated) to a similar temperature is brought into pressurized contact with the coating roller. The coating material deposited on the pre heated piston surface has much of the remaining solvent flashed away, and a thin, solid layer is deposited on the piston surface with each turn. The pressurized contact maintains the thinness of the layer, and also assures good surface adhesion. The piston is coated for as many turns and with as many thin, tightly adhering layers as desired, which build up effectively into a single thicker, homogenous and even layer, with most of the solvent already gone. Each thin layer is deposited quickly, and the piston need not be rolled or otherwise manipulate to prevent sagging before it is sent to the curing oven. The curing oven hardens the final layer, as normally, even though most of the solvent is pre removed, and the part is final ground to size.
The coating material uses a conventional polyamide resin binder and organic solvent, but with a very low, almost negligible percentage of PTFE, so low that the final coating layer would not be considered a solid lubricant coating as such. Instead of soft, low strength PTFE, a higher strength ceramic material, titanium dioxide, is mixed with the resin binder as a primary component. This provides a much harder, stronger final coating layer, which has better compatibility with oil, and better performance at high temperature and loading. In addition, the titanium dioxide component has a smaller differential of thermal expansion relative to aluminum, so as to better maintain surface adhesion at high temperatures.